The present invention relates to the catalytic disproportionation of alkenes. Specifically, this invention relates to an improvement in the catalytic disproportionation of alkenes by use of a distillation column reactor.
According to this invention, the term disproportionation refers to the conversion of a hydrocarbon into similar hydrocarbons of both higher and lower numbers of carbon atoms per molecule. In the case of alkenes, a mixture of new products is obtained comprising alkenes of both higher and lower molecular weights. Such an operation is useful in many instances. For example, a more plentiful hydrocarbon can be converted to a less plentiful and, therefore, more valuable hydrocarbon. One instance of such a conversion occurs when the process of this invention is used to balance the alkene production of a naphtha cracking plant by disproportionating the large quantities of butenes into ethylene and hexene, or propylene and pentene. The disproportionation of butenes is a particularly valuable disproportionation reaction for the use of excess butenes. Approximately equimolar quantities of the higher and lower molecular weight alkenes may be produced by such disproportionation reactions. The higher molecular weight alkenes produced by disporportionation may be cracked to yield additional ethylene or propylene.
Much of the prior art describes conventional processes for the disproportionation of alkenes. Typically, the conventional process is carried out batchwise or in a continuous manner, using the catalyst in the form of a fixed bed, a fluidized bed or a moving bed. At the end of the reaction period, the hydrocarbon phase is separated from the solid catalyst phase and the hydrocarbon products are recovered. Well-known techniques, such as fractional distillation, solvent extraction and adsorption are employed for the separation of the hydrocarbon products.
Conventional processes for the disproportionation of alkenes, such as those described hereinabove, may be operated at temperatures as high as 500.degree. C., but are unable to achieve simultaneous high conversion and high selectivity to desired products. See, e.g., Banks, R. L., J. Molecular Catalysis, V. 8, pp. 269-276 (1980). Alkene disproportionation reactions are reversible; therefore, theoretically the maximum conversion which can be achieved is limited by the thermodynamic equilibrium. In the disproportionation of butene-1 to ethylene and trans-hexene-3, for example, the conversion at equilibrium is about 50 percent. Selectivity is normally controlled by the catalyst and by process conditions, such as temperature, pressure and residence time. In a fixed bed reactor, the residence time is determined by the feed rate. In the disproportionation of butene-1, for example, a slow feed rate will result in a longer residence time for the propylene product, eventually leading to propylene disproportionation and the production of ethylene. On the other hand, a high feed rate will result in a short residence time for propylene and hence less ethylene in the product mix. Unfortunately, high feed rates reduce conversion; thus it is difficult to produce high conversion and high selectivity to propylene simultaneously. In addition, despite the numerous methods of controlling process conditions, many conventional disproportionation processes give broad product distributions, including the by-products of isomerization and secondary disproportionation reactions. Several illustrations of the prior art and its inherent limitations are presented hereinbelow.
U.S. Pat. No. 3,261,879 (1966) discloses a disproportionation of olefin hydrocarbons by contact with a catalyst containing molybdenum oxide or tungsten oxide in a conventional reactor. Conversions are taught to vary over a wide range; however, at high conversion a broad distribution of C.sub.2-12 olefinic products is shown. Isomerization yields are taught to be high.
U.S. Pat. No. 3,463,827 (1969) discloses a process for the disproportionation of olefins by contact with a Group VIB metal carbonyl associated with alumina, silica or silica-alumina. The process is conducted in a fixed bed reactor, and the products are separated in a fractionation column. Conversions are taught to be low, less than 20 percent for butene-1, and isomerization tends to be high.
U.S. Pat. No. 3,448,163 (1969) describes a conventional process for the disproportionation of olefins employing a catalyst comprising rhenium heptoxide impregnated on alumina. In the disproportionation of butene-1, the combined selectivity to ethylene and hexenes is taught to be 93 percent at a conversion of only 29 percent.
U.S. Pat. No. 3,641,189 (1972) discloses a conventional olefin disproportionation process utilizing a rhenium heptoxide catalyst supported on alumina. High selectivities are accompanied by low conversion of the feedstock. U.S. Pat. No. 3,642,931 (1972) teaches a conventional olefin disproportionation process employing a catalyst comprising rhenium heptoxide supported on a refractory oxide of zirconium, thorium, tin, or mixtures thereof. The degree of disproportionation is taught to be less than 15 percent.
U.S. Pat. No. 3,676,520 (1972) discloses a method of disproportionating olefins by contacting the olefin with a catalyst comprising rhenium oxide and a support, such as alumina. The process is carried out in any of the aforementioned standard reactors. In the disproportionation of propylene, the conversion is taught to be less than 5 percent at high contact temperature. There are no teachings on how to control the selectivities of higher olefins, such as butenes.
In view of the deficiencies of the prior art methods, it would be desirable to possess a process for the disproportionation of alkenes which would be capable of achieving simultaneous high conversion and high selectivity at moderate temperatures. It would also be desirable to possess a process for the disproportionation of butenes, such that a high yield of ethylene or propylene, and the corresponding hexenes or pentenes, could be achieved by a simple adjustment of the operating conditions. Such a process would easily meet the demands for varying olefin feedstocks. It would also be desirable to obtain the disproportionation and the separation of products simultaneously, since any reduction in the number of process steps offers considerable economic advantages.